I'm impressed with all the rusty frame repairs. Each rusty frame pic affected the structural integrity of the frame but repairs with welded metal patches appear to be adequately reinforced. I would prefer welding larger areas to ensure an end to the problem. Treat with anti-rust treatment for a permanent solution.
I did something similar to the STMech repair with a couple of changes
1) Used a 4" Square tube 1/8" thick 5 1/2 long and sawn length wise down the middle to make 2 square U channels as available U channels were not the correct shape inside. Still had to splay them out a little to fit, did this by jamming in a 4x4 and 1/2 plywood. This approach also didn't require any bench welding but may not work if cross beam rust goes up higher
2) Added the MDX rear subframe braces per other threads for extra support (sliver bar from side frame to subframe mount bolt). To enable this I used a 10" 1/2" bolt, again a Grade 5 because I couldn't locate a Grade 8 one.
3) Used a 4 1/4" Steel tube 1" OD .219" thick, to sleeve 1/2" bolt inside of cross beam for added support
4) Blue under support is a few 1/2 fender washers tapes together for ease of insertion to replicate the original lower round cross member lower surface that was gone.
5) This picture is the test fit, plan to remove the saddle and use some 3M panel bonding epoxy inside saddle to seal and bond. Then I'll add 2 #12 SS self tapping screws through the holes along the top of the saddle.
1) Removing the original bolts was challenging, Used a propane torch thru the rust hole to heat up the threaded insert really hot, held it with a vice grip and removed the bolt using a 1/2 pneumatic impact driver.
2) The right side wasn't as bad, the original threaded insert was still firmly in the cross member so added the saddle to help hold the bolt to the cross member sides and replaced the original Honda bolt with the Acura Bolt, Special (14X125) 90168-S3V-000 which has a lower threaded end for the rear subframe brace.
I'm original owner of this 2004 Pilot with 290,000 miles but in very good shape and near new tires so wasn't ready to junk it yet. This was an under $100 repair for the metal, epoxy, rustproofing. I'll be keeping a close eye on this for safety but seems really solid right now.